...Buying the airport extreme would be a waste of money. The FIOS router is a G router, unless they have a newer model that has N. Since your connection from the FIOS box to the router is coax cable and not ethernet, you would have to connect the airport extreme to the FIOS router. That is G out from FIOS and G in to airport. You cannot get N out from a G in unless someone can explain to me how you get more out of less in. Thats like trying to get 20 amps out of a 15 amp circuit breaker.
You connect the FiOS router to the Airport Extreme by ethernet cable. An ethernet connection is faster than 802.11n wireless connection. You will get true 802.11n speed from the AE because the FiOS router is basically just a cable modem in this configuration and the AE is the router.